An endovascular stent graft is designed to exclude the flow of blood to an aneurysm that has been formed within the wall of the lumen (for example, the aorta). This is achieved by accessing the aneurysm via an artery, usually within the patient's leg, with a system designed to deliver, position and deploy the stent graft so that it bridges and seals off the aneurysm.
In order to deliver a stent graft to the locus of the aneurysm, it is usually collapsed (that is, reduced in diameter), loaded in a delivery system (where it is retained in the collapsed configuration by a catheter sheath) and delivered to the aneurysm where it is positioned and deployed by expanding its diameter to seal off the aneurysm as described above.
The stent graft delivery system, which contains the stent graft, is inserted into the patient over a guide wire that has already been placed into the patients' arterial system. This type of ‘over-the-wire’ technique is very well known in endovascular surgery (performing vascular surgery from within the vessel) and permits a variety of catheter-based instruments and devices to be placed into the patient's arteries by passing them over the same guide wire. Examples of such instruments and devices include diagnostic catheters, angioplasty and molding balloons, snares, stents, occluders and endovascular stent grafts.
Once the stent graft is in position over the guidewire, deployment of the stent graft is carried out by the surgeon operating a series of control wires (for example) to manipulate the stent graft remotely in order to move it into the correct position (both longitudinally and rotationally) and to control its shape. The stent graft may then be deployed by partially withdrawing the catheter sheath to enable the stent graft to expand in diameter at the locus of the aneurysm. The control wires pass from a control mechanism operated by the surgeon through the delivery catheter and are connected to the stent graft. It is necessary to employ a valve in the sheath which allows a plurality of control wires and a variety of combinations of guidewires, stent grafts and other delivery system components to pass therethrough but which prevents the patient's blood from leaking out of the delivery sheath.
Preferably the delivery system is designed in such a way that the control wires and other mechanisms can be removed completely from the patient through the sheath and valve, leaving the sheath and valve behind in the patient's artery. This will provide an access conduit for other instruments or devices to be passed through the valve and sheath and into the patient's arteries, without losing blood through the sheath and without the need to replace the sheath with another catheter or sheath. In this way, the valve described herein has a first application as an essential component of a stent graft delivery system and a second application as a component of a valved vascular access sheath. Those skilled in the art will be able to identify other applications of the valve.
Various valves are disclosed in EP 0550069 A1 (Guy); US 2011/251565 A1 (Malewicz); WO 98/17341 A2 (Mayo Foundation); WO 03/048616 A1 (Cook, Inc); and US 2010/036504 A1 (Sobrino-Serrano et al.).
US 2010/0224802 A1 (Mialhe) discloses a valve for medical instruments which includes a cylindrical passage which can be at least partially sealed by twisting the valve in order to deform torsionally a flexible section of the passage wall. The disadvantage of this system is that it requires positive intervention from the surgeon in order to seal the valve, in addition to all of the other tasks which the surgeon needs to undertake.
U.S. Pat. No. 7,753,952 B2 (Mialhe) discloses a similar valve with similar short comings. Other valves of this type are disclosed in U.S. Pat. Nos. 7,445,623, 8,118,275 and 6,808,520 (all in the name of the same applicant).
FR 2,863,504 (Mialhe) discloses a similar valve to those discussed above which can be fixed in the closed position (under torsion) but which is releasable with user intervention. In particular, radial grooves secure the movable part in the angular position.
The principal disadvantage with these prior art valves is that they require surgeon intervention in order to operate the valve. There are additional problems however such as achieving a good seal when the guidewire is approximately the same diameter as the internal diameter of the valve hole (when the valve is in the “sealed” position). Prior art valves also have difficulty dealing with a wide range of diameters of guidewires or other instruments and devices when they are introduced through the valve and into the sheath.
The majority of prior art valves use a silicone seal which flexes in order that the valve hole can be dilated when guidewires are inserted into the valve. The expansion range of the silicone seal is an important property in determining the sealing characteristics of the valve. In general, silicone has an expansion range of 500-1000%, which means that a 1 mm hole can expand to a diameter in the range of 5-10 mm. In order to operate effectively in a stent graft delivery system, a valve is required to provide a seal around guidewire(s) ranging from 0.9 mm to 6 mm in external diameter. This requires a valve having a hole with an internal diameter of 0.5 mm (to provide an effective seal around a 0.9 mm guidewire) and an expansion up to 6 mm. However, this requires an expansion ratio of 1200%, which is not possible with prior art silicone valves.